There has been recently proposed to provide a vehicle with a tire pressure monitoring system (TPMS), as means for monitoring whether the air pressure filling a tire (internal pressure) is normal or abnormal. In this instance, a sensor module for measuring the internal pressure or internal temperature of the tire, etc., is provided inside the tire. The measured data is transmitted from the sensor module by wireless communication and received by a receiver on a vehicle body side, so that the received data is displayed by an on-board display device in a driver's cabin.
In this type of tire pressure monitoring system, the sensor module provided within a tire is usually driven by battery. Thus, if the battery is exhausted and becomes unable to operate, it is impossible for the sensor module to transmit the measured data of the tire to the receiver on the vehicle body side. Therefore, it is highly desirable to suppress the electric power consumption of the battery for the sensor module as far as possible, and various proposals have been made to that end.
Patent Document 1 discloses a tire information acquisition system having a measuring/transmitting mode in which the measurement and data transmission are performed at a constant time interval, and a sleeping mode wherein the processing and transmission are halted. Also, Patent Document 2 discloses a tire pressure monitoring system that is switched into a low power consumption mode in a parking state of the vehicle.                [Patent Document 1] JP 2004-314727A        [Patent Document 2] JP 2004-322927A        